Field of the Invention
The invention relates to a method for operating a gas-turbine and steam-turbine plant including a waste-heat steam generator disposed downstream of a gas turbine on the exhaust-gas side and having a high-pressure preheater connected into a water/steam circuit of a steam turbine with a low-pressure part. The invention also relates to a plant operating according to the method.
In a gas-turbine and steam-turbine plant, heat contained in an expanded working medium from the gas turbine is utilized for generating steam for the steam turbine. The heat is transmitted through the use of a number of heating surfaces which are disposed in the form of tubes or tube bundles in a waste-heat steam generator located downstream of the gas turbine on the exhaust-gas side. The heating surfaces are themselves connected into the water/steam circuit of the steam turbine. The water/steam circuit includes a plurality, for example two or three, pressure stages, with each pressure stage having a preheater, an evaporator and a superheater.
In order to achieve as high a plant efficiency as possible during the transmission of heat, the configuration of the heating surfaces within the waste-heat steam generator is adapted to the temperature trend of the exhaust gas. Thus, in a three-pressure process with intermediate superheating, the so-called three-pressure IS process, an especially high steam-turbine power and consequently an especially high overall efficiency of the plant are achieved for a predetermined gas-turbine power. A gas-turbine and steam-turbine plant working according to the three-pressure IS process is known from European Patent 0 436 536 B1. However, even in that known plant, the overall efficiency is limited to about 55%.